The present invention realtes to generally a color television receiver and more particularly a video amplifier therefor combined with a matrix circuit for combining the luminance signal with the color difference signals.
In general, the Y or luminance signal of the color television or colorplexed video signal is transmitted with a wide band while the chrominance signal, with a narrow band. The luminance signal and the carrier chrominance signal are recovered by the luminance and chrominance signal processing circuits, respectively, in the television receiver, and are combined in a matrix circuit to derive the primary color signals which control the intensities of the beam currents in a picture tube for the reproduction of the picture in color.
In the prior art video amplifier combined with the matrix circuit of the type described above, the color difference signal amplifier comprises a common-emitter transistor with the feedback and collector output capacitances of the order of 1.5 to 2.5 PF. The high-frequency component of the luminance signal applied to the collector is negatively fed back so that the high-frequency components of the primary color signal derived from the collector are lost. That is, the color difference signal amplifier must be so designed that even though its function is to amplify the color difference signal transmitted with a narrow band, it must minimize the loss of the high-frequency components of the luminance signal. To overcome this problem, it has been proposed to connect a Darlington circuit consisting of transistors to the base of the transistor in the color difference signal amplifier. In other words, a low-output impedance conversion circuit is connected to the color difference signal amplifier so as to minimize the effects due to the collector feedback capacitance of the transistor in the color difference signal amplifier. However the above circuit arrangement is very complex. Moreover the emitter of the transistor in the color difference signal amplifier is grounded so that the DC operating point drifts due to the ambient temperature variation. Therefore it is not desirable to directly couple the output of the video amplifier to the picture tube. Especially the -G primary color signal changes in the direction opposite to that of the other -R and -B primary color signals due to the temperature drift of an inverter so that the white or color balance in the picture tube is adversely affected.